In a mobile communication system including a plurality of cells, when a user equipment (UE) moves from one cell to another, the user equipment switches to the other cell to continue communications. The switching to the other cell is referred to as “handover”.
Typically, when the user equipment moves to a neighboring cell from a serving cell and signal strength in the neighboring cell is higher than signal strength in the serving cell (cell in which the user equipment originally performs communications), the user equipment performs the handover to the neighboring cell.
Specifically, the user equipment performs the handover according to procedures illustrated in FIG. 1.
First, in S1, the user equipment measures signal power of the neighboring cell. Then, the user equipment confirms whether or not the signal power of the neighboring cell satisfies the following Expression 1.Signal power of neighboring cell>Signal power of serving cell+Offset  (Expression 1)
When the Expression 1 is satisfied, the user equipment reports the event (Event A3) to the network (base station apparatus) in S2.
Note that the offset is a value provided so that the handover does not frequently occur from the serving cell to the neighboring cell at a cell boundary. The offset may be either a positive value or a negative value. Generally, the positive value is used as the offset value provided such that the handover does not frequently occur.
In S3, when the network receives the event (Event A3), the network determines that the user equipment should perform the handover to the cell for which the event (Event A3) has been reported, and then executes the handover procedures (S3).
Note that the above-described event is defined as the “Event A3”, but may be defined as any other event, that is, the event other than the “Event A3”.
In LTE (Long Term Evolution) which succeeds a Wideband Code Division Multiple Access (WCDMA) or a High Speed Downlink Packet Access (HSDPA), for example, a signal power in the above-described example may be “Reference Signal Received Power (RSRP)” which is a received power of a reference signal.
The RSRP is defined in a Non-Patent Literature 1. Further, the above-described LTE may be referred to as “E-UTRA/E-UTRAN”. In addition, the reference signal may be a common reference signal, more specifically.
Note that, in the above-described example, the handover is performed based on the RSRP of the serving cell and the RSRP of the neighboring cell. However, a “Reference Signal Received Quality (RSRQ) may be used instead of the RSRP. Here, the RSRQ represents a value expressed as the RSRP divided by a Received Signal Strength Indicator (RSSI) and is defined in the Non-Patent Literature 1. That is, the RSRQ is calculated by the following Expression 2.RSRQ=RSRP/RSSI  (Expression 2)
The RSSI represents the sum of the received powers, that is, a total received power, of all signals such as a desired signal from the serving cell, an interference signal from the neighboring cell, or a noise signal due to a thermal noise and is defined in Non-Patent Literature 1. The RSSI may be referred to as “E-UTRA carrier RSSI.”
Generally, the value of the above-described RSSI differs in different frequency carriers. For example, the value of RSSI is large in the frequency carrier of high congestion degree, while the value of RSSI is small in the frequency carrier of low congestion degree. In this case, the values of RSRQ may sometimes be different by different values of RSSI due to the congestion degree or the like even though the values of RSRP are the same. Therefore, the RSRQ is used at the time of performing the handover of different frequencies, for example.
Note that, the above-described RSRP or RSRQ may be used not only in the above-described “Event A3” but also in other events. In addition, “RS SIR” which is an SIR of the reference signal may be used instead of the above-described RSRP or RSRQ. Further, as a whole, the above-described RSRP, RSRQ, or RS SIR may be referred to as a wireless quality, a quality of wireless signal, or a “Radio quality”.
The radio quality used in the above-described handover has a large effect on the communication quality of the mobile communication system. Particularly, measurement accuracy in S1 is related to the quality of handover.
More specifically, when the measurement accuracy is bad and the radio quality of the neighboring cell is reported with worse than an original value, the handover is not performed in an area where the handover should actually be performed and the communication is disconnected.
Alternatively, when the measurement accuracy is bad and the radio quality of the neighboring cell is reported better than the original value, the handover is performed in an area where the handover must actually not be performed and the communication is disconnected.
That is, in a case where the measurement of the radio quality can be performed with good accuracy, it is possible to perform the handover appropriately, thereby preventing a failure of the handover.
Incidentally, the RSSI used for calculating the above-described RSRQ is measured in only the OFDM symbols containing the reference signal, as illustrated in FIG. 2.
That is, in FIG. 2, the OFDM symbols #0/#4/#7/#11 are the OFDM symbols containing the reference signal, and other OFDM symbols are the OFDM symbols not containing the reference signal.
Note that the reference signal is a reference signal of an antenna port 0, when a plurality of transmission antennas is present.
This is because, when a desired signal from the serving cell or an interference signal from the neighboring cell is not present at the time of measuring the RSSI using the OFDM symbols not containing the reference signal and a power of thermal noise is very small compared to that of the reference signal, an operation in which the RSSI acting as a denominator of the RSRQ is close to “0” and the value of RSRQ diverges to infinity is prevented.
Incidentally, “e-ICIC (Enhanced Inter Cell Interference Coordination)” is under consideration as one technique of LTE or LTE Advanced in 3GPP, the e-ICIC technique being a technique for improving throughput by suppressing the interference from the neighboring cell. In such e-ICIC, as illustrated in FIG. 3, the user equipment performs the measurement of the above-described RSRP and RSRQ on a specific sub-frame.
In FIG. 3, the neighboring cell, which is an interfering cell, does not transmit a signal of downlink to sub-frames #2/#3/#6/#7, for example. In this case, the user equipment for performing the communication using the serving cell to be interfered performs the measurement of the RSRP and the RSRQ on only the sub-frames #2/#3/#6/#7 to which the signal of downlink is not transmitted.
Thus, the interference is present by performing the measurement of the RSRP and the RSRQ, and it is possible to perform the measurement of the RSRP and the RSRQ in which the influence of interference is excluded at the time of performing interference coordination by the e-ICIC.
Note that, for example, from a network by a signaling of RRC, the user equipment is notified that to which sub-frame the signal of downlink from the neighboring cell is not transmitted, that is, on which sub-frame the user equipment performs the measurement of the RSRP and the RSRQ.